1. Field of the Invention
The present invention relates to a signal detection apparatus and a signal detection method in a radio station in an environment in which plural radio systems share the same frequency band.
2. Description of the Related Art
In many cases in the current radio communications, a specific frequency band is assigned to each radio communication system in order to avoid mutual interference. However, in recent years, in order to efficiently utilize limited frequency resources in radio communications, a method is being studied for using the same frequency band in plural communication systems. A cognitive radio technique is known as a technique for allowing plural radio systems to coexist in the same frequency band. According to this technique, radio environment is recognized so that communication is performed by setting transmission parameters such as a center frequency, a signal bandwidth, beam pattern and the like based on the recognition result and a rule for sharing frequencies and the like. Especially, in the environment in which plural radio systems share a frequency, it is necessary that each cognitive radio apparatus recognizes use status of radio resources before starting transmission as accurately as possible in order to improve frequency use efficiency while avoiding interference to the primary system and avoiding inference between cognitive systems.
As known techniques for recognizing a signal, there are power detection, matched filter detection, feature detection and the like. As a representative example for using signal reception level, the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) scheme is known. For example, this technique is described in Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer(PHY) specifications, ANSI/IEEE Std 802.11, 1999 Edition (to be referred to as non-patent document 1). The wireless LAN system represented as the IEEE802.11 standard uses the CSMA/CA scheme, so that the wireless LAN system determines availability of data transmission by measuring reception level before performing data transmission. In addition, in techniques disclosed in Japanese Laid-Open Paten Application No. 2006-222665 (to be referred to as patent document 1) and A. V. Dandawate and G. B. Giannakis, “Statistical tests for presence of cyclostationarity,” IEEE Trans. Signal Processing, vol. 42, no. 9, September 1994 (to be referred to as non-patent document 2), presence or absence of a signal is determined by calculating a feature amount of cyclostationarity of the signal, and each radio station performs transmission only when a signal is not detected in order to avoid interference. Such feature detection is a technique using statistical characteristics of the signal, and there are advantages in that advance information such as bandwidth and frame format and the like is not necessary and that synchronization of time and frequency is not necessary.
By using these techniques, even when different radio communication systems share the same frequency band, a radio station can detect presence of a signal used in the environment so that the radio station can transmit a signal when it is determined that interference does not occur.
In the CSMA/CA scheme described in the non-patent document 1, since presence or absence of a signal is determined based only on received power, it is unknown that the detected signal is what signal. Also, in the signal detection by using the matched filter, it is necessary to prepare a template of a signal to be detected in a receiver, and that the feature of the detection target signal is completely known. Therefore, if the feature is unknown, there is a problem in that even when a radio resource is available, the radio resource cannot be used.
On the other hand, as shown in the non-patent document 2, in the feature detection utilizing the statistical characteristics of the signal, it is possible to detect respective signals having different features based on small amount of advance information. But, there is a problem in that calculation load required for signal detection becomes very heavy as the number of types of detection target signals increases. In addition, there is a problem in that, when reception levels are different for different types of signals in the receiver, the feature of a weak signal is buried in the feature of a strong signal, so that detection rate of a weak signal is substantially lowered.